In an optical recording-reproducing device for recording information and reproducing the same by applying a laser beam onto a recording medium such as an optical disk, a laser beam of a relatively higher level than that used for reproducing is generally used when recording is conducted. As a result of this, the surface conditions of the disk are changed for the purpose of achieving recording, while a low-level laser beam is used for reading information from the optical disk when reproducing is conducted. In order to record and reproduce information by means of such laser beams, a focusing servo system for correctly aligning the focal point of the laser beam on the surface of the disk and a tracking servo system for correctly making the focal point of the laser beam follow the recording tracks, even if the surface of the disk deflects, are needed.
When, for example, focus control is performed by means of such a servo system, a photodetector 2 is, as shown in FIG. 1, provided in such a manner that it receives a light beam reflected from the disk 1, for the purpose of detecting any focus error. Then a detection signal obtained by the photodetector 2 is amplified by a pre-amplifier 3, and gain and phase compensations are applied to the signal by a gain and phase compensating circuit 4. The detection signal is then input to a power amplifier 5. In response to an output from the power amplifier 5, an electrical current is passed through an actuator 6 formed by an electromagnetic drive means or the like for vertically and horizontally moving an optical system. As a result of this, focus control is performed. A triangular wave generator 7 acts to made the actuator 6 move the optical system in a sawtooth manner in order to move the optical system for the purpose of first aligning the focal point of the optical system with the disk surface. A switch circuit 8 is driven by an output signal from a level detecting circuit 9 and is switched in such a manner that it supplies the triangular signal to the actuator 6 while the optical system is being moved, while a signal from the gain and phase compensating circuit 4 is supplied to the actuator 6 after the movement of the optical system has been completed. An amplifier 10 amplifies the detection signal from the photodetector 2, and the thus-obtained output is applied to the level detecting circuit 9.
In order to obtain a focus error signal or tracking error signal from the detection signal output from the photodetector, a photodetector formed by a PIN photodiode divided into, for example, four pieces is employed. This error signal detector is, as shown in FIG. 2, constituted by two operational amplifiers 11 and 12, each adding detection currents I.sub.A, I.sub.B, I.sub.C and I.sub.D supplied from corresponding photodiodes, and an operational amplifier 13 for subtracting the added value. Each error signal is obtained using the output signal from the operational amplifier 13 for subtracting.
The magnitude of the output of the laser beam changes by a factor of approximately 10 from when information is, as described above, written onto a recording medium and when information is read from the recording medium. Furthermore, the detected quantity of light changes by a factor of 100 or more due to the changes in the reflectance of the recording medium. Therefore, in the above control system in which an error signal obtained from the conventional simple operational amplifier is used, the focus error signal and tracking error signal cannot be accurately obtained if the magnitude of the detection signal obtained by the photodetector is the minimum level.
In order to solve the above-described problem, a control system has been disclosed in Japanese Patent Laid-Open No. 248235/1986 in which an error detection output obtained from a detection signal from the photodetector is divided by a detection output (sum of the quantity of light signal) corresponding to the sum of the quantity of reflected light obtained from the detection signal from the photodetector, for the purpose of obtaining a focus error control signal or a tracking error control signal. As a result of this, a control system is achieved which provides control in proportion to error and which does not depend on the quantity of reflected light.
In order to achieve the above-described division, a system is disclosed in which the switching control circuit is controlled by a sum of the quantity of light signal so as to switch the gain for the purpose of standardizing error signals. Furthermore, a feedback method is disclosed in which an analog multiplier is used. The analog multiplier is provided with an AGC circuit and a gain control amplifying circuit in such a way that the gain control amplifying circuit is controlled by the gain control voltage of the AGC circuit for the purpose of performing the division.
However, the conventional dividing means must have a complicated circuit, and a large number of expensive external parts such as a capacitor to provide a time constant for the AGC circuit must be provided. Furthermore, adjustment of the characteristics to delete offset is difficult to conduct, and a wide dynamic range is impossible to obtain.